Power supply systems having a chargeable and dischargeable secondary battery that can supply an electric power to a load device and also can be charged when needed. Typically, this kind of power supply systems are mounted on hybrid vehicles, electric vehicles and the like that use an electric motor driven by the secondary battery as a drive power source. For example, the electric vehicle uses the power stored in the secondary battery for driving the vehicle. The hybrid vehicle is driven by the electric power that is driven with the electric power stored in the secondary battery, and is also driven while assisting the engine by the electric motor. A fuel cell vehicle is driven by the electric motor that is driven with the electric power provided from a fuel cell, and is also driven with the electric power stored in the secondary battery in addition to the electric power of the fuel cell.
Particularly, in the hybrid vehicle of the type that generates an electric power by driving a power generator by an internal combustion engine mounted on the vehicle and can charge the secondary battery with the power thus generated, it is necessary to control a State Of Charge (SOC) of the secondary battery to attain substantially an intermediate state (50%-60%) between a fully charged state (100%) and a completely discharged state (0%) so that the secondary battery can receive a regenerative electric power and, when necessary, can immediately supply the electric power to the electric motor.
When the secondary battery is overcharged or overdischarged, this deteriorates a battery performance to reduce its life. Therefore, when the secondary battery is used while repeating the charge and discharge for attaining a control target, i.e., the intermediate SOC as described above, it is necessary to grasp successively the state of charge of the secondary battery and to perform the charge/discharge control for restricting the excessive charging and discharging.
Therefore, Japanese Patent Laying-Open Nos. 2003-346919, 2000-100479, 11-204149 and 2000-268886 (patent documents 1, 2, 3 and 4, respectively) have proposed control structures that grasp, from a macroscopic viewpoint, a battery state based on charge/discharge currents, an output voltage (terminal voltage), and further grasp the battery state based on dynamic estimation of an internal state of the secondary battery including various factors such as an ion concentration distribution in an active material forming the secondary battery. Particularly, in connection with a lithium ion battery, W. B. Gu and C. Y. Wang, “THERMAL-ELECTROCHEMICAL COUPLED MODELING OF A LITHIUM-ION CELL”, ECS Proceedings Vol. 99-25 (1), pp. 743-762 (non-patent document 1) has disclosed a battery model equation that enables estimation of an electrochemical reaction inside the battery.